1. Field of the Invention
The present invention relates to a plasma addressed liquid crystal display device having a plasma cell and a liquid crystal cell stacked one above the other by way of a dielectric sheet. More specifically, it relates to a method of manufacturing a plasma cell by using a screen printing process.
2. Description of Related Art
A constitution of a plasma addressed liquid crystal display device will be explained briefly with reference to FIG. 7. The plasma addressed liquid crystal display device is disclosed, for example, in Japanese Patent Laid-Open Hei 4-265931. As illustrated in the figure, the plasma addressed liquid crystal display device has a structure comprising a lamination of a liquid crystal cell 101, a plasma cell 102 and a dielectric sheet 103 interposed between both of them. It is necessary that the dielectric sheet 103 is as thin as possible for driving the liquid crystal cell 101, and an extremely thin plate glass with a thickness of about 50 .mu.m is used. The liquid crystal cell 101 is constituted within an upper glass substrate 104 and signal electrodes D are formed in a stripe pattern on the main inner surface thereof. The substrate 104 is bonded to the dielectric sheet 103 at a predetermined gap using spacers 105. A liquid crystal layer 106 is filled in the gap. The gap has a size of usually of about 5 .mu.m, which has to be kept uniformly over the entire display surface.
On the other hand, the plasma cell 102 is constituted with a lower glass substrate 107. A plurality of plasma electrodes 108 are formed in a stripe pattern on the main surface of the glass substrate 107. The glass substrate 107 is joined by way of a frit seal member 109 to the dielectric sheet 103. An ionizable gas is sealed in a space sealed by the frit seal member 109. A barrier rib 110 is formed on each of the plasma electrodes 108 by means of a screen printing process. The plasma cell 102 is divided by barrier ribs 110 into a stripe shape to constitute electric discharge channels. The screen printing process is a simple technique capable of forming a fine pattern and remarkably improving the productivity and the working efficiency.
By the way, while the barrier ribs 110 are formed by screen printing, they require a considerable thickness since they function as a gap spacer for the plasma cell 102. However, the height of the barrier ribs 110 varies, and unevenness such as screen mesh residue is also caused to the top portion of individual barrier ribs. Accordingly, in a state where the top portion of the barrier ribs 110 and the dielectric sheet 103 made of extremely thin plate glass are in contact with each other, undulation is caused to the surface of the dielectric sheet, so that flatness can not be maintained. As a result, the thickness of the liquid crystal layer 106 on the side of the liquid crystal cell 101 can not be controlled uniformly to remarkably deteriorate display quality. In addition, the distance of the gap also varies between the lower glass substrate 107 and the dielectric sheet 103, failing to obtain uniform plasma discharge.
In view of the above, it has been adopted a countermeasure of previously printing and baking the barrier rib 110 to a somewhat larger thickness and then subsequently planarizing by polishing. However, the barrier rib 110 usually has a width of about 100 .mu.m and a height of 100 to 300 .mu.m. Since the height is larger as compared with the width, the mechanical strength of the rib is weak and, particularly, the end of the barrier rib is fragile. Therefore, when polishing is applied for planarization of the top portion after printing and baking, there is a problem that the barrier ribs are broken or destroyed under the effect of mechanism stresses.